Stereochemistry reduction with alkali metals

Reduction of the A -bond in compound (lOl) with alkali metals and alcohols formed the C14 center of asymmetry. It was found that the stereochemistry of this reaction depends fundamentally both on the reagents used and on the substituent at Cn. Thus, on reduction, lljS -hydroxy derivatives form only the 14/3 -epimers while 11-ketones form mixtures of the two possible epimers. The best results were achieved by the preliminary reduction of (101) to the corresponding llck -hydroxy derivative and the reaction of the latter with potassium and isopropanol in liquid ammonia. This led exclusively to the desired 14o -epimer (100) [93, 685-689]. 

Double bonds conjugated with benzene rings are reduced electrolytically [344] (p. 23). Where applicable, stereochemistry can be influenced by using either catalytic hydrogenation or dissolving metal reduction [401] (p. 24). Indene was converted to indane by sodium in liquid ammonia in 85% yield [402] and acenaphthylene to acenaphthene in 85% yield by reduction with lithium aluminum hydride in carbitol at 100° [403], Since the benzene ring is not inert toward alkali metals, nuclear reduction may accompany reduction of the double bond. Styrene treated with lithium in methylamine afforded 25% of 1-ethylcyclohexene and 18% of ethylcyclohexane [404]. 

The main methods of reducing ketones to alcohols are (a) use of complex metal hydrides (b) use of alkali metals in alcohols or liquid ammonia or amines 221 (c) catalytic hydrogenation 14,217 (d) Meerwein-Ponndorf reduction.169,249 The reduction of organic compounds by complex metal hydrides, first reported in 1947,174 is a widely used technique. This chapter reviews first the main metal hydride reagents, their reactivities towards various functional groups and the conditions under which they are used to reduce ketones. The reduction of ketones by hydrides is then discussed under the headings of mechanism and stereochemistry, reduction of unsaturated ketones, and stereochemistry and selectivity of reduction of steroidal ketones. Finally reductions with the mixed hydride reagent of lithium aluminum hydride and aluminum chloride, with diborane and with iridium complexes, are briefly described. 

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